refactor: replace next-auth with better-auth core and client

src/app/api/auth/[...all]/route.ts

@@ -0,0 +1,4 @@
+import { auth } from "@/auth";
+import { toNextJsHandler } from "better-auth/next-js";
+
+export const { GET, POST } = toNextJsHandler(auth);

src/app/api/auth/[...nextauth]/route.ts

@@ -1,2 +0,0 @@
-import { handlers } from "@/auth";
-export const { GET, POST } = handlers;